Ignition Timing Theory

What ignition advance does

The ignition spark fires before the piston reaches top dead centre (TDC). This lead time is the ignition advance, measured in degrees of crankshaft rotation before TDC (deg BTDC).

Combustion is not instantaneous. After the spark, the flame front takes time to propagate across the combustion chamber. The goal is for peak cylinder pressure to occur a few degrees after TDC (roughly 15-20 deg ATDC on most engines), where the piston geometry gives the best mechanical advantage on the crankshaft.

If the spark fires too late (retarded), combustion is still developing as the piston descends and the expansion ratio is wasted. Power and efficiency fall.

If the spark fires too early (advanced), peak pressure arrives before the piston reaches TDC. The burning mixture pushes against a rising piston, increasing pumping work and generating heat rather than torque.

MBT: maximum brake torque

MBT (maximum brake torque) is the advance angle that produces the highest torque at a given RPM and load. It is the optimum timing in the absence of knock.

MBT varies considerably across the operating range:

• At light load (low MAP, small throttle opening), the charge is dilute with residuals and burns slowly. More advance is needed, often 35-45 deg BTDC at light cruise.
• At high load (high MAP, approaching WOT), the charge is dense and burns faster. Less advance is needed, typically 15-28 deg BTDC at WOT on a mild road engine.
• At low RPM, there is more time available for combustion. Less advance is needed.
• At high RPM, the available time per cycle shrinks. More advance is needed, but the faster gas velocity partially compensates.

Knock (detonation)

Knock is uncontrolled auto-ignition of the end-gas (the unburnt charge ahead of the flame front) before the flame reaches it. The rapid pressure rise creates a characteristic metallic knock or ping. Sustained knock causes piston crown erosion, head gasket failure, and eventually catastrophic engine damage.

The knock threshold is the maximum advance that can be used before detonation occurs. It is always equal to or less than MBT. On a healthy engine with good fuel, knock threshold and MBT are close; on a compromised engine (carbon deposits, poor cooling, low-octane fuel), knock threshold may be well below MBT.

Octane rating (RON/MON) measures a fuel's resistance to knock. Higher octane fuel tolerates more advance before detonating. Running higher-octane fuel on a well-tuned engine allows the timing to be advanced closer to MBT, recovering power and efficiency that would otherwise be sacrificed for safety margin.

The ignition advance table

Like the VE table, the ignition advance table is a 2D map indexed by RPM and load. Each cell contains the advance in degrees BTDC.

A starting point for a mild naturally aspirated road engine:

• Idle (600-900 RPM, low MAP): 10-15 deg BTDC
• Light cruise (1500-2500 RPM, 30-60 kPa MAP): 30-40 deg BTDC
• WOT at low RPM (1500-2500 RPM, 90-100 kPa MAP): 18-25 deg BTDC
• WOT at mid RPM (3000-4000 RPM, 90-100 kPa MAP): 22-30 deg BTDC
• WOT at high RPM (5000+ RPM, 90-100 kPa MAP): varies widely with engine

These are starting points only. Always start conservative (less advance than you think you need) and work towards MBT carefully, listening for knock and watching coolant temperature.

Knock sensitivity and safety margin

Tuning towards MBT requires a safety margin below the knock threshold. How much depends on how consistently the engine can be expected to stay cool, the fuel quality available, and whether a knock sensor is fitted.

Without a knock sensor, use a generous margin (3-5 deg below the point where knock is first heard). With a knock sensor and active retard, the ECU can pull timing when knock is detected, allowing the table to run closer to the edge.

Never tune WOT timing without monitoring coolant temperature. An overheating engine will knock at advance angles that are safe when cold.

Cranking and idle advance

Cranking advance is typically 5-10 deg BTDC. More advance during cranking makes starting harder; too little and the engine fires weakly. This is usually a fixed value rather than a table.

Idle advance has an outsized effect on idle quality. Too little advance at idle and the engine runs rough and hot. More advance at idle raises idle speed; the ECU can use advance as a fast-idle correction, trimming timing back to lower idle RPM, which is faster to respond than adjusting injector pulse width.

Dwell

Dwell is the time the coil primary circuit is energised before the spark fires, during which the coil stores energy. It is measured in milliseconds (or degrees on some ECUs).

Too little dwell and the coil does not fully charge; the spark is weak, causing misfires, particularly at high RPM where the available time between events is short.

Too much dwell wastes energy, overheats the coil, and can damage the coil driver in the ECU. At high RPM, excessive dwell can overlap with the next charging cycle.

Correct dwell is specified by the coil manufacturer, typically 2-4 ms. It should be compensated for battery voltage: at low voltage (cold cranking), the coil charges more slowly and needs more time to reach full saturation.

Ignition advance and fuelling interaction

Advance and fuelling interact. Retarding timing at WOT is sometimes used to manage exhaust temperature on turbocharged engines; more retard increases exhaust enthalpy and spools the turbo faster, at the cost of some efficiency. This is a deliberate tuning choice, not a problem to fix.

Conversely, a rich mixture (low lambda) burns more slowly, which shifts the MBT angle towards more advance. If VE table corrections change the mixture significantly, the ignition table may need revisiting.